The sources of noise in a readback signal from a magnetic recording medium have been investigated and identified. One of those sources includes the irregularities and defects in the microstructure of the magnetic medium itself. For many years, the noise generated from this source has been thought, as with the noise generated from other identified sources, to be random and subject only to statistical analysis for its treatment. The inventors herein have recently demonstrated that this noise component is instead deterministic, i.e. is permanent and repeatable, depending entirely on the head-medium position and on the magnetic history of the medium. As confirmed by experiments conducted by the inventors herein, when the medium has had no signal written on it and has been recorded only with DC fields, the observed readback signals are almost identical. The magnetic contribution to the readback signal under these conditions results from spatial variations in the medium's magnetization: magnetic domains, ripple, local fluctuations of the anisotropy field and saturation magnetization. These local properties, in turn, are affected by the morphology and magnetic properties of the individual grains which make up the domain and which do not change after deposition. Hence, the noise from a nominally uniformly magnetized region measured at a fixed position on a magnetic medium is reproducible. As shown by the inventors herein, a magnetic medium may be DC saturated and its output then measured to determine the noise of its remanent state. The inventors have confirmed that this remanent noise is a function of the magnetic microstructure by comparing the remanent noise after a positive DC saturation with the remanent noise after a negative DC saturation. It has been found that these wave forms are virtual "mirror images" of each other thereby demonstrating a close correlation. Similarly, other methodologies were used to confirm that the remanent noise was determinative, repeatable, and related to the physical microstructure of the magnetic medium itself. Remanent noise arising from the permanent microstructure exhibits identifiable features characteristic of that permanent microstructure (magnetic fingerprint) after practically any magnetic history. See Spatial Noise Phenomena of Longitudinal Magnetic Recording Media by Hoinville, Indeck and Muller, IEEE Transactions on Magnetics, Volume 28, No. 6, November 1992, the disclosure of which is incorporated herein by reference.
There is a long felt need in the art for a method and apparatus to use this magnetic fingerprint as a means for control for the magnetic medium as it is manufactured and used, i.e. manipulated, throughout its life. There is a wide variety of pre-recorded magnetic media presently being marketed and/or distributed in the United States and throughout the world. Examples of these magnetic media include those produced and sold in the entertainment industry including magneto-optic disks and tapes, cassette tapes, reel-to-reel tapes, video tapes, etc. Still another major market in magnetic media is the tremendous volume of computer programs routinely sold and/or distributed on floppy diskettes. A related market is the magnetic media used in hard disk drives which have met with great commercial success. All of these examples of magnetic media have to be first manufactured and, subsequently, recorded with data which are later retrieved by reading with a magnetic head of some kind. A typical manufacturing process for a thin film magnetic medium would include taking a substrate, such as a Mylar.TM. film, depositing magnetic material thereon, such as by coating it with a slurry, magnetizing and drying the slurry, and then calendering the substrate, and slitting and rolling the substrate into usable format. After deposition, a fingerprint may be taken and used as a benchmark to control subsequent steps in processing. With this process control, better quality medium may be manufactured. As an example, various steps may actually cause damage to the magnetic media and it is important to ascertain such damage as soon as possible in order to eliminate waste and reduce cost through unnecessary further processing thereof. Furthermore, once the magnetic medium has been manufactured and sold, and placed in service, a user would like to ensure that the magnetic medium is still "good" or that it is being processed without deformation thereof which would corrupt or lose data between recording and playback. An example of the former is when a hard disk drive experiences a "head crash" which can corrupt some or all of the data stored on a hard disk, much to the chagrin of a user. An example of the latter is perhaps best exemplified by playback of analog recorded audio cassette tapes wherein the tape is maintained under tension during both record and playback which, if not consistent, can distort the signal retrieved from the cassette and the sound reproduced thereby.
In order to solve these and other problems in the prior art, the inventors herein have succeeded in developing a method and apparatus for utilizing the unique, deterministic, remanent noise characteristic of the magnetic medium itself as a fingerprint or benchmark to ascertain the integrity of the magnetic medium as it is both processed in manufacturing and later used in the recording and playback of data therefrom. As used herein, the word "data" may include any kind of magnetic recording, analog or digital, and may typically include words, numbers, video, and audio.
This inventive technique relies upon the discovery that the microscopic structure of the magnetic medium itself is a permanent random arrangement of microfeatures and therefore deterministic. In other words, once fabricated, the recording medium's physical microstructure remains fixed for all conventional recording processes. In particulate media, the position and orientation of each particle does not change within the binder for any application of magnetic field; in thin film media, the microcrystalline orientations and grain boundaries of the film remain stationary during the record and reproduce processes. It is the magnetization within each of these fixed microfeatures that can be rotated or modified which forms the basis of the magnetic recording process. If a region of a magnetic medium is saturated in one direction by a large applied field, the remanent magnetization depends strongly on the microstructure of the medium. This remanent state is deterministic for any point on the recording surface. Each particle or grain in the medium is hundreds to thousands of Angstroms in dimension. Due to their small size, a small region of the magnetic surface will contain a very large number of these physical entities. While the fabrication process normally includes efforts to align these particles, there is always some dispersion of individual orientations, positions and magnetic properties. The actual deviations will be unique to a region of the medium surface making this orientation a signature or a "fingerprint" of that medium. To reproduce this distribution, intentionally or not, is practically impossible since this would entail a precise manipulation of the orientation of innumerable particles at the submicrometer level. Thus, the orientation and disposition of a large set of particles on a specific portion of a recording surface can uniquely identify that medium. In experiments, the inventors have found that the remanent noise from a length of between about 30 micrometers and 4300 micrometers provides enough data to "fingerprint" a magnetic medium uniquely.
In the manufacturing process, once this "fingerprint" has been established and becomes part of the magnetic medium, it can be used as a benchmark which may later be checked during subsequent steps in the manufacturing process in order to control it and ensure the integrity of the magnetic medium. For example, with continuous tape magnetic medium, a wide web of tape may first be formed and the web then subsequently processed by slitting and calendering into a desired width after which it may be cut to specific lengths and packaged in a final case, carrier, or the like for actual use. Early on in this process, and intermittently throughout the web, a "fingerprint" of a small portion thereof may be determined and stored for later correlation with a "fingerprint" read from the same portion thereof. If the cross-correlation between the two "fingerprints" is high, one can be assured that the integrity of the magnetic medium has not been disturbed. However, if the cross correlation is less than high, then it is highly likely that there has been some change in the magnetic medium which, if not within tolerance, could be indicative of a defective magnetic medium. The original or "benchmark" fingerprint may be stored separately or may itself be magnetically encoded on the magnetic medium at or adjacent to the portion thereof whose fingerprint has been taken.
Subsequent to manufacturing, a magnetic fingerprint may be used in much the same way to monitor the magnetic medium. However, at this stage in its use, the magnetic medium may be reliable enough that the fingerprint may instead be useful in monitoring other parts of the magnetic circuit which are involved in reading and/or writing data to and from the magnetic medium. For example, in a hard disk drive, a fingerprint for a portion of the magnetic hard disk may be magnetically encoded thereon and the head of the hard disk drive may be tested by reading the fingerprint and comparing it with the magnetically encoded, stored fingerprint. A cross-correlation between these two fingerprints may be used to determine how effectively the head is functioning in reading data to and from the magnetic medium of the hard disk. Within certain tolerances, this cross-correlation may produce an error signal which can be used by the electronics in the hard disk drive to "adapt" the head output in order to effectively adjust the head response for wear, etc. Out of tolerance, this cross-correlation may be used to generate an error signal which could warn of impending head failure or other failure in the magnetic circuit.
Still another application of the fingerprint is its use in a magnetic medium comprised of a linear tape which is commonly "tensioned" as data are either read to or written therefrom. As with the other applications, the magnetic fingerprint may be magnetically encoded on the tape itself and cross-correlated with the same magnetic fingerprint as it is read in real time. The cross-correlation may be used to determine the nature of the variation between the fingerprints, i.e. such as a fixed expansion of the fingerprint, which would be indicative of tension being too great in the playback mode. The tension control of the playback device may thus be adjusted to decrease its tension and thereby assure that the same tension is applied to the tape during read as well as write functions. This helps ensure the integrity of the data and its accurate recovery from the magnetic medium.